The present invention relates to a composite fire sensor operable in response to fire phenomena such as flame, heat, smoke and gas.
Fire sensors used in Japan (i.e., fire sensors recognized by the Fire Prevention Law in Japan) are of two kinds, heat sensors and smoke sensors. The heat sensors are of constant temperature type, differential type, compensation type, etc. Constant temperature type and differential type heat sensors employ a bimetal, which consists of a metal of a low thermal expansion coefficient and a metal of a high thermal expansion coefficient, these metals being bonded to each other. The bimetal is secured at one end, and when it receives heat, it is curved toward the low thermal expansion coefficient metal, that is, its free end is displaced in proportion to temperature change. The sensor thus operates such that a switch is closed when a constant temperature is reached by the bimetal temperature.
The smoke sensors are of photoelectric type and ion type. The former photoelectric type smoke sensors include light reduction type and scattering type smoke sensors. FIG. 9 shows a light reduction type smoke sensor. Its light-emitting section 1 includes a light source 3, a lens 4 and a light-receiving element 2 for compensating for light intensity reduction of the light source. Its light-receiving section 10 is disposed at a suitable distance from the light-emitting section 1 and includes a lens 11, a throttle 12 and a light-receiving element 13. When smoke 6 comes to the light path 5 emitted from the light-emitting section 1, the dose of light incident on the light-receiving element 13 is reduced by an amount proportional to the amount of smoke present in the light path. When the dose of incident light is reduced to a predetermined value, the sensor is actuated. The flame sensors have a purpose of early detection of fire, and are recognized as fire sensors by the UL Standards in U.S.A. and the NFPA. This type of fire sensors include ultraviolet sensors and infrared sensors and also include visible light sensors proposed earlier by the applicant, which can detect both flame and smoke with a single sensor element (Japanese Patent Application NO. 58-69,752).
The gas sensors typically include contact combustion type sensors and semiconductor type sensors. The semiconductor type sensor utilizes variation of the electic conductivity of semiconductor with a phenomenon of gas absorption taking place on the surface of a metal oxide (e.g., SnO.sub.2 and ZnO). FIG. 10 shows the structure of a gas sensor. As shown, it has electrodes 15 embedded in a metal oxide semiconductor piece 16. One of the electrodes is used as a heater, while the other electrode is used for measuring the electric resistance of the portion of the semiconductor between the two electrodes. The heater is provided to heat the sensor to a temperature (200.degree. to 400.degree. C.) at which the gas adsorption and desorption can readily take place on the surface of semiconductor. The prior art sensors for sensing heat, smoke, flame and gas as noted above, however, only serve the respective roles independently, and there is no relation at all among them.